1. Field of the Invention
This invention relates generally to a method for cooling the combustion chamber and venturi used in a gas turbine engine for reducing nitric oxide emissions and to a structure for improved cooling effectiveness of a venturi throat region. Specifically a method is disclosed for cooling the combustion chamber/venturi to lower nitric oxide (NOx) emissions by introducing preheated cooling air into the premix chamber for use in the combustion process.
2. Description of Related Art
The present invention is used in a dry, low NOx gas turbine engine typically used to drive electrical generators. Each combustor includes an upstream premix fuel/air chamber and a downstream combustion chamber separated by a venturi having a narrow throat constriction that acts as a flame retarder. The invention is concerned with improving the cooling of the combustion chamber which includes the venturi walls while at the same time reducing nitric oxide emissions.
U.S. Pat. No. 4,292,801 describes a gas turbine combustor that includes upstream premix of fuel and air and a downstream combustion chamber.
U.S. Pat. No. 5,117,636 and U.S. Pat. No. 5,285,631 deal with cooling the combustion chamber wall and the venturi walls. The patents state that there is a problem with allowing the cooling air passage to dump into the combustion chamber if the passage exit is too close to the venturi throat. The venturi creates a separation zone downstream of the divergent portion which causes a pressure difference thereby attracting cooling air which can cause combustion instabilities. However, it is also essential that the venturi walls and combustion chamber wall be adequately cooled because of the high temperatures developed in the combustion chamber.
The present invention eliminates the problem discussed in the prior art because the cooling circuit for the venturi has been adjusted such that the cooling air no longer dumps axially aft and downstream of the venturi throat into the combustion zone. In fact, cooling air flows in the opposite direction so that the air used for cooling the combustion chamber and the venturi is forced into the premix chamber upstream of the venturi, improving the efficiency of the overall combustion process while eliminating any type of cooling air recirculation separation zone aft of the venturi as discussed in the U.S. Pat. No. 5,117,636.
Recent government emission regulations have become of great concern to both manufacturers and operators of gas turbine combustors. Of specific concern is nitric oxide (NOx) due to its contribution to air pollution.
It is well known that NOx formation is a function of flame temperature, residence time, and equivalence ratio. In the past, it has been shown that nitric oxide can be reduced by lowering flame temperature, as well as the time that the flame remains at the higher temperature. Nitric Oxide has also been found to be a function of equivalence ratio and fuel to air (f/a) stoichiometry. That is, extremely low f/a ratio is required to lower NOx emissions. Lowering f/a ratios do not come without penalty, primarily the possibility of xe2x80x9cblow-outxe2x80x9d. xe2x80x9cBlow-Outxe2x80x9d is a situation when the flame, due to its instability, can no longer be maintained. This situation is common as fuel-air stoichiometry is decreased just above the lean flammability limit. By preheating the premix air, the xe2x80x9cblow-outxe2x80x9d flame temperature is reduced, thus allowing stable combustion at lower temperatures and consequently lower NOx emissions. Therefore, introducing the preheated air is the ideal situation to drive f/a ratio to an extremely lean limit to reduce NOx, while maintaining a stable flame.
In a dual-stage, dual-mode gas turbine system, the secondary combustor includes a venturi configuration to stabilize the combustion flame. Fuel (natural gas or liquid) and air are premixed in the combustor premix chamber upstream of the venturi and the air/fuel mixture is fired or combusted downstream of the venturi throat. The venturi configuration accelerates the air/fuel flow through the throat and ideally keeps the flame from flashing back into the premix region. The flame holding region beyond the throat in the venturi is necessary for continuous and stable fuel burning. The combustion chamber wall and the venturi walls before and after the narrow throat region are heated by the combustion flame and therefore must be cooled. In the past, this has been accomplished with back side impingement cooling which flows along the back side of the combustion wall and the venturi walls where the cooling air exits and is dumped into combustion chamber downstream of the venturi.
The present invention overcomes the problems provided by this type of air cooling passage by completely eliminating the dumping of the cooling air into the combustion zone downstream of the venturi. The present invention does not permit any airflow of the venturi cooling air into the downstream combustion chamber whatsoever. At the same time the present invention takes the cooling air, which flows through an air passageway along the combustion chamber wall and the venturi walls and becomes preheated and feeds the cooling air upstream of the venturi (converging wall) into the premixing chamber. This in turn improves the overall low emission NOx efficiency.
An improved method for cooling a combustion chamber wall having a flame retarding venturi used in low nitric oxide emission gas turbine engines that includes a gas turbine combustor having a premixing chamber and a secondary combustion chamber and a venturi, a cooling air passageway concentrically surrounding said venturi walls and said combustion chamber wall. A plurality of cooling air inlet openings into said cooling air passageway are disposed near the end of the combustion chamber.
The combustion chamber wall itself is substantially cylindrical and includes the plurality of raised ribs on the outside surface which provide additional surface area for interaction with the flow of cooling air over the combustion cylinder liner. The venturi walls are also united with the combustion chamber and include a pair of convergent/divergent walls intricately formed with the combustion chamber liner that includes a restricted throat portion. The cooling air passes around not only the cylindrical combustion chamber wall but both walls that form the venturi providing cooling air to the entire combustor chamber and venturi. As the cooling air travels upstream toward the throat, its temperature rises.
The cooling air passageway is formed from an additional cylindrical wall separated from the combustion chamber wall that is concentrically mounted about the combustion chamber wall and a pair of conical walls that are concentrically disposed around the venturi walls in a similar configuration to form a complete annular passageway for air to flow around the entire combustion chamber and the entire venturi. The downstream end of the combustion chamber and the inlet opening of the cooling air passageway are separated by a ring barrier so that none of the cooling air in the passageway can flow downstream into the combustion chamber, be introduced downstream of the combustion chamber, or possibly travel into the separated region of the venturi. In fact the cooling air outlet is located upstream of the venturi and the cooling air flows opposite relative to the combustion gas flow, first passing the combustion chamber wall and then the venturi walls. The preheated cooling air is ultimately introduced into the premix chamber, adding to the efficiency of the system and reducing nitric oxide emissions with a stable flame.
The source of the cooling air is the turbine compressor that forces high pressure air around the entire combustor body in a direction that is upstream relative to the combustion process. Air under high pressure is forced around the combustor body and through a plurality of air inlet holes in the cooling air passageway near the downstream end of the combustion chamber, forcing the cooling air to flow along the combustor outer wall toward the venturi, passing the throat of the venturi, passing the leading edge of the venturi wall where there exists an outlet air passageway and a receiving channel that directs air in through another series of inlet holes into the premix chamber upstream of the venturi throat. With this flow pattern, it is impossible for cooling air to interfere with the combustion process taking place in the secondary combustion chamber since there is no exit or aperture interacting with the secondary combustion chamber itself. Also as the cooling air is heated in the passageway as it flows towards the venturi and is introduced into the inlet premix chamber upstream of the venturi, the heated air aides in combustor efficiency to reduce pollutant emissions.
The outer combustor housing includes an annular outer band that receives the cooling air through outlet apertures upstream of the venturi. The air is then directed further upstream through a plurality of inlet air holes leading into the premix chamber allowing the preheated cooling air to flow from the air passageway at the leading venturi wall into the premix area.
The combustion chamber wall includes a plurality of raised rings to increase the efficiency of heat transfer from the combustion wall to the air, giving the wall more surface area for air contact. Although a separate concentric wall is used to form the air cooling passageway around the combustion chamber and the venturi, it is possible in an alternative embodiment that the outer wall of the combustor itself could provide that function.
In an alternate embodiment of the present invention, a venturi is disclosed that includes a throat region and incorporates a cooling passageway having a reduced cross sectional area proximate the throat region to provide improved cooling effectiveness. The venturi also incorporates a plurality of raised ridges spaced at a predetermined distance from the venturi throat region and from adjacent raised ridges along the cooling passageway such that the raised ridges disturb the cooling flow passing through the passageway, and when used in conjunction with the reduced cross sectional area proximate the throat region, provide a more effective heat transfer mechanism at the venturi throat region.
It is an object of the present invention to reduce nitric oxide (NOx) emissions in a gas turbine combustor system while maintaining a stable flame in a desired operating condition while providing air cooling of the combustor chamber and venturi.
It is another object of this invention to provide a low emission combustor system that utilizes a venturi for providing multiple uses of cooling air for the combustor chamber and venturi.
And yet another object of this invention is to lower the xe2x80x9cblow-outxe2x80x9d flame temperature of the combustor by utilizing preheated air in the premixing process that results from cooling the combustion chamber and venturi.
And yet a further object of this invention is to provide a gas turbine combustion system utilizing a venturi with a cooling passageway that provides improved cooling to a venturi throat region through cooling passageway geometry changes.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.